Trekking in Nepal can be undertaken throughout the year depending on the season of the year and region of your trek. The tourist arrival pattern suggests that Nepal has two main trekking seasons - From February to May and late August to December. Especially, the period between September to December is considered to be the main tourist season in Nepal.
However, trekking is possible at any time of the year if you choose the destination according the time of your visit. Perhaps, the most uncomfortable period is the rainy season from the middle of June to the middle of September. The monsoon rains cover most part of southern plains and middle mountains during this period. Some people like the botanists, and those interested in rain forests find even this period a memorable experience. Even for trekkers, the Trans-Himalayan regions of Mustang and Dolpo may be an option which remains free from the monsoon activity.
Autumn: September-November Festival and Harvest Time, ideal trekking period, tourist season of Nepal During the high tourist season in October and November, flights and hotels are fully booked and hotels and trails in the hills can be pretty busy.
Winter: December-February ideal for trekking at lower elevations, generally below 3000 meters. Winter is milder at lower elevations, and it rarely snows below 2000 meters.
Spring: March-May Good Mountain views during over 4000 meters and temperature is quite moderate. The trails are covered with wild flowers and rhododendrons in full bloom, the rivers begin swelling from the melting Himalayan snows.
Summer & Rain: June-August Summer months really continue up to mid-September making travel wet and warm. Trek on the rain-shadow areas north of the Himalaya like Mustang and Dolpo.